The potential gradient that induces movement of ions into hair cells includes both the resting potential of the hair cells and the positive potential of the endolymph. As noted previously, the total gradient across the apical membrane of hair cells is about 140 mV. Therefore, a change in K+ conductance in the apical membranes of hair cells results in a rapid current flow that produces the receptor potential in these cells. This current flow can be recorded extracellularly as a cochlear microphonic potential, an oscillatory event that has the same frequency as the acoustic stimulus. The cochlear microphonic potential represents the sum of the receptor potentials of a number of hair cells.
Hair cells, like retinal photoreceptors, release an excitatory neurotransmitter (probably glutamate) when depolarized. The neurotransmitter produces an excitatory postsynaptic potential (EPSP) in the cochlear afferent nerve fibers with which the hair cell synapses. In summary, sound is transduced when oscillatory movements of the basilar membrane cause transient changes in the transmembrane voltage of the hair cells and, finally, the generation of action potentials in cochlear afferent nerve fibers. The activity of a large number of cochlear afferent fibers in the auditory nerve can be recorded extracellularly as a compound action potential.
On the basis of differences in width and tension, investigators originally concluded that different parts of the basilar membrane have different resonant frequencies.
•Fig. 8.19 Different Frequencies of Sound Result in Different Amplitudes of Displacement at Different Sites Along the Organ of Corti. A, Travelingwaveproducedinthebasilarmembranebyasoundof200Hz.Thecurvesata,b,c,anddrepresentdisplacementofthebasilarmembraneatdifferenttimes,andthedashed line istheenvelopeformedbythepeaksofthewaveatdifferenttimes.Maximumdeflectionoccursatabout29mmfromtheovalwindow.B, Envelopesoftravelingwavesproducedbyseveralfrequenciesofsound.Notethatthemaximumdisplacementvarieswithfrequencyandisclosesttothestapeswhenthefrequencyishighest.(RedrawnfromvonBekesyG.Experiments in Hearing. NewYork:McGraw-Hill;1960.)
For example, the basilar membrane is about 100 µm wide at the base and 500 µm wide at the apex. It also has higher tension at the base. Thus, the investigators predicted that the base would vibrate at higher frequencies than would the apex, as do the shorter strings of musical instruments. Experiments have shown that the basilar membrane moves as a whole in traveling waves (
Fig. 8.19 ), but displacement of the basilar membrane is maximal nearer the base of the cochlea during high-frequency tones and maximal nearer the apex during low-frequency tones.
In effect, the basilar membrane serves as a frequency analyzer; it distributes the stimulus along the organ of Corti, and different hair cells respond differentially to particular 50 Hz frequencies of sound. This is the basis of the place theory of hearing. In addition, hair cells located at different places along the organ of Corti may be tuned to different frequencies because of variations in their stereocilia and biophysical properties. As a result of these factors, the basilar mem brane and organ of Corti have a so-called tonotopic map (
Fig. 8.20).
Neurotransmitter release by hair cells in the organ of Corti can evoke action potentials in the primary afferent fibers of the cochlear nerve. Afferent fibers in the vestibulocochlear nerve (CN VIII) are bipolar cells with a myelin sheath around the cell bodies, as well as around the axons. The cell bodies are in the spiral ganglion, their peripheral processes synapse at the base of hair cells, and their central processes 400 Hz synapse in the cochlear nuclei of the brainstem.
acteristic frequency. The characteristic frequency can be determined from a tuning curve for the fiber (